根据下面资料，回答题
Walking, if you do it vigorously enough, is the overall best exercise for regular physical activ-ity. It requires no equipment, everyone knows how to do it and it carries the（46）risk of in-jury. The human body is designed to walk. You can walk in parks or along a river or in your neighborhood. To get（47）benefit from walking, aim for 45 minutes a day, an average of five days a week.
Strength training is another important（ 48）of physical activity. Its purpose is to build and（49）bone and muscle mass, both of which shrink with age. In general, you will want to do strength training two or three days a week,（50）recovery days between sessions.
Finally, flexibility and balance training are（ 51）important as the body ages. Aches and pains are high on the list of complaints in old age. The result of constant muscle tension and stiff-ness of joints, many of them are（52）, and simple flexibility training can（53）these by making muscles stronger and keeping joints lubricated (润滑). Some of this you do whenever you stretch. If you watch dogs and cats, you´ ll get an idea of how natural it is. The general（54）is simple: whenever the baby has been in one position for a while, it is good to（55）stretch it in an opposite position.
A．allowing
B．avoidable
C．briefly
D．component
E．determined
F．helping
G．increasingly
H．lowest
I．maintain
J．maximum
K．prevent
L．principle
M．provoke
N．seriously
O．Topic
第（46）题选